Could the difference of this season be attributed to the fact that GRRM did not write any episodes for this season?
Well the episodes he wrote tended to be good but he only ever did one episode per season, and I don't think one episode would make the difference. Unless him not writing an episode was indicative of a rift between him and the producers and he wasn't consulting them in general, which is possible, but I don't think that's it.
My guess it is a combination of the fact there are now so many storylines and characters in different locations so that it often feels like very little happens in a single episode (while simultaneously feeling like lots of storylines don't have much time to breathe), the fact that to reduce the storylines they are combining storylines or making up more non-book material which can lead to inconsistencies and some characters feeling more directionless, and just the fact that the source material they have to work with is weaker. I like A Feast For Crows and A Dance With Dragons very much but the things that are good about them are the things that don't translate very well into the TV show, where as the stuff that works well in the TV show is what is lacking in them (which is probably why you often see people arguing they are too long or boring). Compare that with A Storm Of Swords, usually regarded as the best, which contains the subtle and compex world building, foreshadowing and character development which the TV show can't do as well, but also has a ton of plot development and big events, which is the thing that the TV show does well and sometimes even better than the book. Even in condensed form and with original material added, there have been a lot less major events in the first half of this season than in Season 4, where big events occurring compensated somewhat for not spending much time on each story per episode. Jon Snow taking up command at the Wall, Daenerys struggling to rule in Meereen, Cersei plotting against the Tyrells, people travelling to and setting up for events in Winterfell - these are all pretty slow burners, with the biggest events so far not exactly measuring up to the purple wedding early in season 4. Also quite importantly fan favourite Tyrion had a much less compelling plot in travelling through Essos (and despite being two of the wittiest characters, Tyrion and Varys pairing didn't really live up to the hype since they were lacking in things to scheme about). Tyrion's scenes have picked up a bit now that he is paired up with Jorah, especially after episode 5. I'm holding out hope that the last few episodes will be better than the first part of the season, because I think that a lot of the big moments that serve as payoff for this season really are left to the end. Episode 8, 9 and 10 have every right to be crackers.
Also I think it can't be forgotten that the show has lost a few more key characters like Tywin, Joffrey, the Hound. Killing off major characters is great for good episodes and storylines, but it does mean that you lose a lot of great characters and actors that helped make the show what it was, and over time that will add up and may leave the show feeling lacking. Ned Stark, Catelyn Stark, Robb Stark are gone, and while Tywin, Joffrey or the Hound may not be quite as main as them they were some of the things that made the show really entertaining.
Lastly I agree that the Dorne storyline has been bad. I can't really work out how it ended up so poor. The Sand Snakes are cheesy in the book, with their "Each one looks different and uses a different weapon" type thing, but in the show they've combined the cheesiness and each using their own weapon with them all wearing the same outfit and having a really awful introduction scene, so now they are a cheesy team that are barely differentiable, even though their differences are meant to be their gimmick. Thinking about it, I also think budget limitations are showing in the Dorne scenes. The Water Gardens set is excellent, but it seems to be the only location they have. Which is why we have Jaime and Bronn landing on a coastline that looks suspiciously like Ireland, staying on the beach for their fight and travelling since at least it has sand, and of course the unforgettable random tent in a patch of sand where we met the Sand Snakes. That is also probably why the confrontation had to take place in the Water Gardens, which I think contributed to it being so ridiculous (Jaime and the Sand Snakes both being able to sneak in extremely easily, just walking up to Myrcella with no real plan, and breaking into a fight where it will obviously be noticed in a place full of Doran's guards). I wonder if they knew they wanted to include Dorne to capitalise on the popularity of Oberyn, but didn't really have a good idea for the storyline for it and so we ended up with a rush job.
There's a chance that Dorne might also be better in the rest of the season, since it will presumably now be able to focus on dialogue between Jaime, Bronn, the Sand Snakes (their fight scene was awful but two of them had barely had any lines yet so I can't yet judge them too harshly on acting in actual conversations) and of course Doran Martell, who seems well done in his brief scenes. I'm not sure it has any real potential though - for me in the books the best part of Dorne was not really Arianne Martell (who was cut from the show to be kind of replaced by the Sand Snakes), definitely not the Sand Snakes, but Doran himself. But what made him great was that despite appearing weak and passive he had a plan, but the things relating to his plans (
Quentyn going to Daenerys, and currently Arriane heading to meet with the invading (f)Aegon) seem to be cut from the show, so I am not sure what he can really do.
Seeing Dorne and also how much of a stretch it is to include all the existing storylines makes me rethink my hope for including the Iron Islands storyline
Especially since I'm sure I recall that scenes with ships are some of the most expensive scenes.